The clinical use of dapsone has expanded over the last decade. Dapsone is now the drug of choice for certain autoimmune-associated disorders such as dermatitis herpetiformis, and is an important component of prophylaxis and chemotherapy for opportunistic infections, such as P. carnii pneumonia, in AIDS patients. In both situations, dapsone's hemotoxicity; viz, methemoglobinemia and hemolytic anemia, are dose limiting in therapy. Over the life of this grant, we have used a rat model to identify the hemotoxic metabolites (N-hydroxydapsone [DDS-NOH] and N-acetyl-N-hydroxydapsone [MADDS-NOH]) of the drug and to gain insight into the mechanism underlying the hemolytic response. We now propose to test the hypothesis that the insight gained with the rat model can be used to reduce the severity of dapsone-induced hemotoxicity in humans and hence improve the therapeutic ratio of the drug. Four aims are presented. The first will examine whether the mechanistic concepts developed with rat red cells applies to human cells, and if so, the relative responsiveness of human vs rat red cells. Parameters to be examined include heme oxidation (methemoglobin formation); oxy- and/or thiyl-radical production; glutathione and protein mixed disulfide formation (G-SS-G and Pr-SS-G); formation of hemoglobin- skeletal protein adducts and of other membrane-bound hemoglobin monomers and polymers; echinocyte formation; and susceptibility to macrophage ingestion. The second aim will characterize the effect of inhibitors of dapsone N-hydroxylation in rat hepatic microsomal preparations and, for selected inhibitors, on dapsone N-hydroxylation in the acute and chronic rat models of dapsone hemotoxicity, as a prelude to the clinical studies. The third aim will determine if the severity of hemotoxicity in volunteers and patients receiving single doses of dapsone can be correlated with blood levels of DDS-NOH+MADDS-NOH, and with their CYP3A4 phenotypic status. The fourth aim will examine she capacity of inhibitors of CYP3A4 (which has been implicated in the N-hydroxylation of dapsone in humans) to suppress both blood DDS-NOH+MADDS-NOH levels and hemotoxicity. Potential inhibitors to be tested include cimetidine, ketaconazole, grapefruit juice, and trimethoprim. Further studies will determine if the extent of binding of hemoglobin monomers and polymers to membrane components can be used to develop a quantitative assay to assess the severity of a hemolytic response occurring in patients during chronic dapsone therapy.